1. Field of the Invention
This invention relates to semiconductor wafer fabrication, and more particularly to the detection of harmful chemical species within a chamber of a semiconductor wafer processing device.
2. Description of the Relevant Art
In order to eliminate undesirable effects caused by the presence of chemical species normally found in the surrounding air, several semiconductor wafer fabrication operations are performed in a vacuum. Vacuum is a condition of low pressure, generally below standard atmospheric pressure. A vacuum is generally formed by evacuating air molecules from a chamber using a vacuum pump. Wafer fabrication processes performed in a vacuum include evaporation, sputtering, low pressure chemical vapor deposition (LPCVD), and ion implantation.
The first four wafer fabrication processes listed above are deposition processes used to deposit a layer of a select material upon an exposed surface of a semiconductor wafer. The air around us, exclusive of water vapor, is about 78 percent (by volume) nitrogen (N.sub.2), 21 percent oxygen (O.sub.2), and 1 percent other gases. The presence of oxygen and/or nitrogen within a deposition chamber during a deposition process may deleteriously affect the properties of the deposited layer. The air also contains moisture (H.sub.2 O) which may also negatively affect the properties of the deposited layer.
For example, one common material deposited upon semiconductor wafers is aluminum. Aluminum layers are chiefly patterned to form electrical conductors called interconnects. All four deposition processes involve liberating aluminum atoms from an aluminum source and depositing the aluminum atoms upon an exposed surface of a semiconductor wafer. If a large number of oxygen molecules are present within the deposition chamber during the deposition process, a substantial number of the aluminum atoms may combine with the oxygen atoms before reaching the exposed surface of the semiconductor wafer. Compounds thus formed containing aluminum and oxygen, including aluminum oxide (AlO) and aluminum trioxide (Al.sub.2 O.sub.3), would be incorporated into the aluminum layer. Aluminum oxide and aluminum trioxide are dielectric materials, and their presence within the aluminum layer reduces the electrical conductivity of the aluminum layer. Successful deposition of aluminum, and any other select material which readily reacts with molecules found in the air to form undesirable compounds, must therefore be carried out in a vacuum. LPCVD is carried out at pressures down to about 1.0.times.10.sup.-3 torr, while evaporation and sputtering involve pressures down to approximately 1.0.times.10.sup.-9 torr.
A typical deposition system includes a load lock chamber and at least one deposition chamber. The load lock chamber includes a gas-tight door for loading containers of wafers to be processed into the load lock chamber and for unloading containers of processed wafers from the load lock chamber. A gas-tight portal separates the load lock chamber from the deposition chamber. During use, the deposition chamber is continuously maintained at an operating pressure below atmospheric pressure (i.e., under vacuum) while the load lock chamber is cycled between atmospheric pressure and the lower operating pressure for wafer container loading and unloading.
From time to time, deposition systems must be removed from service for preventive maintenance (e.g., scheduled component replacements) or repair operations. Under such conditions, the deposition chamber must often be returned to atmospheric pressure (i.e., "vented"). The venting of the deposition chamber allows air, including oxygen, nitrogen, and moisture, to enter the deposition chamber.
Following completion of the preventive maintenance or repair operations, the deposition chamber must be evacuated and sufficiently purged of the oxygen, nitrogen, and moisture before being returned to service. A period of time is allowed for "recovery" of the deposition system. Lacking any direct way to measure the concentrations of various chemical species within the deposition chamber during recovery, the amount of time allowed for recovery is typically based upon operator experience and/or test wafer results. An operator typically allows a recovery time which, in his or her experience, has not resulted in problems caused by the presence of oxygen, nitrogen, or moisture within the deposition chamber.
Chemical species present on surfaces of the wafers prior to deposition may also deleteriously affect the properties of deposited layers. For example, significant amounts of residual substances associated with photoresist processing, chiefly organic compounds, remaining on surfaces of semiconductor wafers following photolithographic patterning processes are known to negatively affect the properties of layers subsequently deposited upon the surfaces of the wafers.
It would thus be advantageous to have a system for directly measuring concentrations of chemical species within a deposition chamber of a deposition system. Such a measurement system would allow detection of harmful chemical species (e.g, oxygen, nitrogen, moisture, and/or organic compounds) within the deposition chamber in time to take corrective action before substantial product is lost. Such a measurement system would also reduce the ambiguity surrounding deposition system recovery operations, allowing deposition systems to be returned to service as quickly as possible following preventive maintenance or repair operations. The measurement system would also desirably measure residual substances emitted into the surrounding chamber to allow corrective techniques to either reduce those contaminants on the surface prior to processing, or to more accurately remove contaminants from the ambient in-situ.